The present invention relates to dynamic motor braking systems working to control the speed of an electric motor by dissipating energy generated by the motor, and in particular to a braking system using a hybrid control strategy that offers greater braking capacity and improved operation of motor drives when connected in tandem.
Motor drives control the frequency and amplitude of the electrical power applied to an electrical motor to improve motor operation, for example, by improving motor starting and stopping, motor speed and torque control, motor synchronization, load management, and energy efficiency. For this purpose, the motor drive will typically receive three-phase line power and rectify it to produce a DC bus voltage. The DC bus voltage is then received by a set of switching semiconductor devices, typically operating in a class D or switching mode, to synthesize multiphase AC electrical power from the DC bus voltage. The frequency and amplitude of the synthesized power is controlled by controlling the switching of the semiconductor devices.
When a control electrical motor must be slowed, for example against an attached inertial load, the motor may generate (termed “regeneration”) electrical power that appears on the DC bus as an increased DC voltage. This voltage is reduced by a resistor (termed a dynamic braking resistor) shunting the DC bus to dissipate the excess power on the DC bus. Typically this dynamic braking resistor is connected across the DC bus by a solid-state switching device so that it can be removed when braking is not required.
Two principal techniques may be used for controlling the dynamic braking resistor in order to regulate the DC bus voltage. The first technique, termed the “hysteretic control”, connects the resistor across the DC bus when the voltage on the DC bus rises above a first predetermined limit and disconnects the resistor from the DC bus when the voltage drops below a second predetermined limit. The difference between the first and second predetermined limits provides a degree of hysteresis preventing excessive switching when the DC voltage is near a limit.
While hysteretic control is simple, it has several drawbacks. First, the regulation of the DC bus voltage is rather coarse producing a voltage “ripple” that can decrease the effectiveness of the motor drive and increase power dissipation and undesired mechanical vibration. Second, often it is desired to connect the DC buses of several motor drives together (termed shared bus or common bus for DC supply configurations) for improved load sharing. When hysteretic control is used, minor differences in the switching limits of the controls will inevitably result in one dynamic braking resistor turning on at a slightly lower level than the other dynamic braking resistor and thus carrying the full burden of regulation. This unequal load sharing eventually reduces the total braking capacity of the connected drives and can result in significant circulating currents between drives.
One solution to these problems of hysteretic control is the use of “pulse width modulated” or PWM control. Under PWM control, the dynamic braking resistor is connected and disconnected across the DC bus at a high switching rate whose “duty cycle” (the relative proportion of time that the resistor is operating in a shunting capacity) depends on the voltage of the DC bus. As the voltage on the DC bus rises, the duty cycle of the dynamic braking resistor increases drawing more power from the DC bus.
The high switching rate of the dynamic braking resistor in PWM control substantially reduces the ripple in the regulation of the DC bus voltage. Further, because the duty cycle is proportional to the excess voltage (rather than on/off), small differences in the control of the dynamic braking resistor do not result in one dynamic braking resistor carrying the full burden of the regulation and any circulating currents are reduced.
Unfortunately, these advantages to PWM control come at an expense. The high switching rate of the solid-state switching device controlling the dynamic braking resistor, necessary to reduce the ripple of the DC bus voltage, increases the power dissipation of the solid-state switching device. This requires that the solid-state switching device be “de-rated”, limiting the maximum amount of braking energy that can be dissipated before the rating of the solid-state switching device is exceeded.